All aircraft include movable control surfaces for directional control in flight. Such control surfaces can include primary flight control surfaces for general flight path control, as well as various lift and drag devices for take off and landing. Primary flight control surfaces can include ailerons for roll control, elevators for pitch control, and rudders for yaw control. Conventional lift and drag devices can include leading edge slats, trailing edge flaps, and spoilers.
FIG. 1 is a schematic top view of a conventional control surface operating system 101 configured in accordance with the prior art. The prior art system 101 includes a movable control surface 104 (such as an aileron) pivotally attached to a wing 102 about a hinge line 106. Redundant actuators 108 (shown as a first actuator 108a and a second actuator 108b) are operably coupled between the control surface 104 and the wing 102 such that simultaneous extension of the actuators 108 causes the control surface 104 to pivot about the hinge line 106 in a first direction, and simultaneous retraction of the actuators 108 causes the control surface 104 to pivot about the hinge line 106 in a second direction opposite to the first direction.
Each of the actuators 108 receives hydraulic power from an independent hydraulic system 103 for redundancy. The hydraulic systems 103 are essentially identical, and each includes an electro-hydraulic servo valve 112, a solenoid valve 114, and a mode selector valve 116 (shown in a closed or “blocked”) position in FIG. 1). In addition, the hydraulic systems 103 further include a fluid inlet circuit 111 and a fluid outlet circuit 113. The fluid inlet circuit 111 passes pressurized hydraulic fluid from a fluid source through a check valve 105 and a filter 107 to the electro-hydraulic servo valve 112 and the solenoid valve 114. The fluid outlet circuit 113 returns low pressure hydraulic fluid from the electro-hydraulic servo valve 112, the solenoid valve 114, and the mode selector valve 116 to the fluid source.
In operation, a flight control computer 118 receives a control input from the pilot and responds by energizing the solenoid valve 114. When energized, the solenoid valve 114 allows pressurized hydraulic fluid to pass to the mode selector valve 116. This pressurized fluid causes a piston or slider 117 within the mode selector valve 116 to move downward against a spring 119 switching the mode selector valve 116 from the blocked mode shown in FIG. 1 to an active mode. In the active mode, the mode selector valve 116 allows pressurized hydraulic fluid to flow from the electro-hydraulic servo valve 112 to the actuator 108 in response to signals from the flight control computer 118. The flight control computer 118 controls the flow of pressurized hydraulic fluid into and out of the actuator 108 as required to move the control surface 104 in accordance with the pilot's input. Under normal operating conditions, both hydraulic systems 103 respond to control signals from the flight control computer 118 as described above to move the actuators 108 in unison and provide the desired control surface movement.
The conventional control surface actuation architecture described above with reference to FIG. 1 provides redundancy for meeting the requirements of being fail-operative for a first component failure and fail-safe for a second component failure. For example, if one component associated with the first actuator 108a (such as the electro-hydraulic servo valve 112) fails, then the computer 118 sends a signal to the solenoid valve 114 causing the mode selector valve 116 to block the flow from the electro-hydraulic servo valve 112 to the actuator 108a. At the same time, the selector valve 116 enables the first actuator 108a to be operated in a bypass mode such that the control surface 104 can be moved solely by the second actuator 108b. If, however, there is a second component failure (for example, such as if both of the electro-hydraulic servo valves 112 fail) then the computer 118 deenergizes both of the solenoid valves 114 causing both of the mode selector valves 116 to move to the blocked mode as shown in FIG. 1. When the mode selection valves 116 are in the blocked mode, the actuators 108 are held in position. Holding the control surface 104 in position in the event of a double failure such as this prevents the control surface 104 from experiencing aerodynamic flutter, which can lead to structural damage. Thus, the prior art system 101 is fail-operative for a single failure because at least one of the actuators 108 can sufficiently operate the control surface 104 under a single failure such as that described above. Further, the prior art system 101 is fail-safe for a double failure because under such a condition both actuators 108 will lock in position and prevent the control surface 104 from experiencing potentially harmful aerodynamic flutter.
One shortcoming of the prior art control surface operating system 101 is the additional cost associated with providing redundant actuator systems. Another shortcoming is the additional airframe weight that such systems add. In addition to these shortcomings, employing multiple actuators on a common control surface often results in a “force fight” between the two actuators each time they move the control surface. Force fights result from the inevitable differences that exist between the forces applied to the control surface by the two actuators. Force fights can introduce high fatigue cycling on structural members. As a result, such members have to be designed with increased structural weight to carry the increased fatigue loads. Further, force fights can result in an undesirable dead band of control surface movement at or near the valve null position, causing poor control surface positionability and responsiveness. Efforts to reduce force fights between redundant actuators generally increase the complexity of the control systems, which in turn increases the cost and weight of such systems. The added complexity of the control systems leads to further complexity in the flight control software that the flight control computer uses to control the hydraulic systems and reduce the force fight between actuators.
Yet another shortcoming associated with redundant actuator systems is that the sub-system components are typically over-designed. These components are over-designed because one actuator system (i.e. one actuator plus the associated hydraulic system) must be capable of moving the control surface (and the other actuator in bypass mode) without any help from the other actuator system in the event of a system failure. This over-design means that under normal operating conditions when both actuators are operative the aircraft will be providing about twice as much power to the control surface operating system as is actually needed to move the control surfaces. Further, this over-design results in heavier hydraulic and control system components, and the resulting hydraulic power extraction can reduce the overall fuel efficiency of the aircraft.